Beyond those two improvements, I don't think there are any definites on the hardware side. At least, not until another iPhone prototype shows up in a bar in the next few days.
Bigger and bigger and bigger: It's possible the iPhone will get bigger. It's possible that the form factor will remain the same, but the screen will take up more of the iPhone's surface area--extending all the way to the edges, or inching closer to the ear speaker. I must believe that if Apple does change the screen's dimensions, it will do so in a way that doesn't require that developers rejigger their apps' artwork--something they already had to do first with the release of the iPad and later with the advent of the Retina display. Asking developers to create yet another set of artwork for a slightly larger iPhone display strikes me as more trouble than it's worth, so Apple will likely keep the total pixel count the same.
New form factor: This seems almost inevitable, but not quite. Rather than suffer any resurgence of the Antennagate drama, I suspect Apple will tweak the iPhone 5's antenna design some. I wouldn't be shocked--in fact, I would be delighted--if Apple replaced the iPhone 4's glass back, too. It's shatter-resistant, but not shatter-proof, as too many iPhone 4 owners have learned the heart-and-phone-breaking way. And let's not forget, the only thing that Apple loves more than straight lines--which the iPhone 4 is full of--is curves, which the iPhone 4 lacks.
System-wide voice navigation: "Wait," you cry, "we've seen iOS 5 previews for months! There's no voice navigation!" If you hadn't interrupted me, though, I would have explained that we've only seen iOS 5 previews on existing iOS devices, not on new hardware that might feature specific improvements aimed at taking advantage of such capabilities.
One particularly juicy iPhone 5 rumor suggests that--thanks to those sure-bet under-the-hood processor improvements, the iPhone 5 (and the iPhone 5 alone) will support systemwide voice interaction. That might take the form of voice transcription anywhere you can currently use the keyboard, or it might get fancier still: TUAW suggests that the iPhone 5 might understand commands like "Text Dan Moren 'Book a flight to northern California ASAP.'" Boy oh boy, do I want to believe.
Turn-by-turn directions: Android phone owners get turn-by-turn directions for free. iPhone users can buy numerous GPS apps from the App Store, but a built-in offering from Apple would be nice. I don't care much one way or the other, having already plunked down cash for the excellent Navigon app, but keeping feature parity with Android's plusses makes good business sense.
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